Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

Why?

Let's start with the study's more obvious findings.

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1. Virginity restoration. You can't recover your lost virginity. But as libertines age and virgins are born, a country can recover its old virginity rate. In the 2002 NSFG survey, 22 percent of men and women between the ages of 15 and 24 said they had never had sexual contact with another person. But in the latest NSFG survey, taken from 2006 to 2008, that number increased to 27 percent of men and 29 percent of women (Table 7, Page 38). In the broader age pool, the trend is diluted but still shows up: Among people ages 15-44, the percentage reporting zero lifetime opposite sex partners increased by two points among men (Table 4, Page 35) and three points among women (Table 3, Page 34). (In case you're wondering, no, there was no shift in reported homosexuality that would account for this increase.) The percentage of men ages 15-44 who reported only one lifetime female partner also increased by two to three points. So if you thought sexual mores were moving inexorably in the direction of more, earlier, and kinkier activity, think again. Virginity can return, and apparently, it has.

2. Normality and age. At ages 15-17, only 33 percent of females and 32 percent of males say they've had vaginal sex. By ages 18-19, the norm has reversed: 62 percent of females and 66 percent of males say they've done it. And by ages 20-24, 85 percent of women and 82 percent of men have done it. Oral sex follows a similar trajectory: The percentage of females who say they've done it goes from 30 percent at ages 15-17 to 63 percent age ages 18-19, and then to 81 percent at ages 20-24. In the same sequence of age brackets, the percentage of males who say they've done it goes from 35 to 70 to 80. Statistically, both acts are abnormal below age 18 but normal above it.

3. The top of the bottom? Five months ago, I noted that a different sex survey showed a big, long-term increase in anal sex reported by women. But there's no guarantee that this trend will continue. In the NSFG, anal sex, like vaginal and oral sex, becomes far more common as teenagers mature. Between ages 18-19 and 20-24, it doubles in prevalence in both sexes. Still, it peaks at 39 percent among women and 45 percent among men, never crossing the statistical threshold of normality. And the percentage of people ages 15-44 who say they've had anal sex hardly budged from 2002 to 2006-8. (It went from 34 to 35.8 percent among men, and from 30 to 30.7 percent among women—not a significant difference.) So it's possible that the surge of reported anal sex will peter out.

4. The heterosexuality gap. In 2002, among people ages 18-44, men were more likely than women to report being attracted only to the opposite sex. At that time, the gap was six to seven percentage points. In the 2006-8 data, the gap has increased to more than 10 percentage points. Ninety-four percent of men, compared with 83 percent of women, say they're attracted only to the opposite sex. (Table 11, Page 42.) Why? Maybe, as anecdotes suggest, women's sexuality is, on average, more fluid than men's. Maybe the taboo against lesbianism has relaxed more than the taboo against male homosexuality. Either way, the gap bears watching.

5. Bisexuality and experimentation. Among men ages 18-44, homosexuality correlates with sex acts as you'd expect. The gayer the man, the less likely he is to have had any kind of sex with a woman (Table 14, Page 45).

But this pattern doesn't hold among women. Start with vaginal sex. Women who say they're attracted only to men are more likely to report that they've had vaginal sex than are women who say they're equally, mostly, or exclusively attracted to women. That makes perfect sense. But women who say they're attracted mostly to men are even more likely to report having had vaginal sex. Isn't that odd? A woman who's mostly rather than entirely straight presumably diverts some of her sexual energy away from men. How does she end up more likely to have had vaginal intercourse?

You could brush off this oddity by noting that the range of variation among these three groups of women, in terms of whether they've had vaginal sex, is fairly narrow. But shift your attention to oral sex, and the pattern gets sharper. Women who say they're attracted only to men are more likely to report having had oral sex with a man than are women who say they're equally, mostly, or exclusively attracted to women. But women who say they're attracted mostly to men are even more likely to report oral sex with a man. And here, the gap is bigger: Compared with exclusively straight women, mostly straight women are more likely to have had oral sex with a man by a margin of 9 percentage points.

Move on to anal sex, and the pattern gets even stranger. Again, women attracted mostly to men are more likely to say they've had anal sex with a man than are women attracted only to men. And in the case of anal sex, the gap is huge: While 30 percent of the totally straight women say they've had anal sex, 55 percent of the mostly straight women say they've done it. But get this: Among women who say they're attracted equally, mostly, or exclusively to women, the figure is 41 percent. Lesbian and bisexual women are more likely than totally straight women to say they've had anal sex with a man.

If you find this number hard to believe, check it against a related finding in the same table: 48 percent of women who self-identify as gay or bisexual report having had anal sex with a man, compared with 33 percent of women who self-identify as straight. This gap is true of anal sex, but not of vaginal or oral sex.

What could explain this?

The last time I wrote about anal sex, I learned not to generalize too much. In some cases, the key factor might be how much pleasure a woman gets from vaginal sex. That factor could correlate with female heterosexuality and, at the same time, make a woman less likely to try anal sex. In other cases, a history of inconsiderate male partners might correlate both with anal sex and with a woman's statement that she's more attracted to women than to men. Or maybe calling yourself fully heterosexual is simply the default answer of women who have limited or no sexual experience. But each of these theories has trouble accounting for the data on vaginal and oral sex.

Here's my best guess: In many if not most cases, openness to experimentation is driving everything else. Adventurousness makes a woman more likely to try various sex acts with men. And at the same time, it makes her more likely to express interest in other women. This factor would have to be strong enough in bisexual women, on average, to overcome the statistical effects of clear-cut lesbians who do nothing sexual with men.

The unkind way of putting this, from a liberal point of view, is that women who claim to be exclusively heterosexual are, on average, more conventional or uptight than women who report having some interest in other women. These "exclusively" straight women might not even be exclusively straight. They're just obtuse to their wayward feelings, or afraid to admit them. The equally unkind view, from a conservative perspective, is that sexual mores are collapsing in all directions.

To me, this theory does the best job of explaining why the experience gap between mostly and exclusively straight women increases as you go from vaginal to oral to anal sex—and why lesbian and bisexual women exceed exclusively straight women only in the case of anal sex. The kinkier the deed, the more it correlates with a woman's lesbian interest, her awareness of that interest, or her willingness to admit it.

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